The present invention relates to a circuit configuration for an uninterruptible power supply apparatus for computers and so on, and in particular, an uninterruptible power supply apparatus based on a common commercial power supply type or method that supplies power from a commercial power supply in a normal circumstance and supplies power from a storage battery to a load through a power conversion circuit if a problem occurs with the commercial power supply.
FIG. 7 shows a circuit configuration (single-wire diagram) based on the conventional common commercial supply power method. In this method, a commercial-power-supply voltage input through an AC input terminal is normally output directly from an AC output terminal via a switch 1, while a charger 3 charges electricty to a storage battery 2 to provide backup power in case of problems with the commercial power supply.
When an error occurs in the commercial power supply, such as a service interruption or decrease or increase in voltage, the switch 1 is instantaneously switched to an inverter 21 side to supply AC power from the storage battery 2 through the inverter 21, thereby achieving uninterrupted power supply (magazine "OHM", February 1989, pp.33).
Since the conventional uninterruptible power supply apparatus directly feeds power to a load through the switch 1 during the normal commercial power supply, there are following problems.
(1) Since the variation of the commercial power supply voltage directly affects the load, an error in the commercial power supply must be detected as quickly as possible in order to switch to the inverter 21 side. That is, the effect of the voltage variation of the commercial power supply must be minimized.
(2) The power conversion circuit, which is composed of the charger 3 and the inverter 21, includes a DC intermediate circuit, which is composed of an AC/DC conversion circuit (converter) and a DC/AC conversion circuit (inverter). Thus, an electrolytic capacitor is required as a DC input voltage power supply for the inverter, resulting in increasing an apparatus size and considering the expected service life of the capacitor.
Thus, an object of the invention is to provide an uninterruptible power supply apparatus that avoids the direct effect onto a load due to a change of a voltage in the commercial power supply during the normal commercial power supply, while eliminating a DC intermediate circuit when power is supplied from the storage battery, thereby stabilizing the voltage supply to the load, reducing the apparatus dimensions, and increasing the service life.